Electromagnetic system



1930. H. c. HARRISON 1,773,032

ELECTROMAGNETIC SYSTEM Filed Dec. 6. 1923 m r r mm Mme/#05 #207 6/74/7750,

Patented Aug. 12, 193$ PAT-EN HENRY O. HARRISON, OF PORT WASHINGTON, NEW YORK, ASSIGNOR TO WESTERN ELECTRIC COMPANY, IN GOBI'ORATED, OF NEW YORK, N; Y., A CORPORATION OF NEW YORK ELECTROMAGNETIC s zsrma Amalication filed December 6, 1923. Serial No. 678,935.

This invention relates to electro-mechanical devices for the transmission and'conver-- sionof oscillatory Wave energy, and 'particularly to that type of device which is required to deal with oscillations of a wide range of frequencies, as, for egzample, in the electrical reproduction of seiindvavesyhy telephone instruments and the like.

The object of the invention is to improve the efliciency of the energy transmission and conversion in such devices, and to maintain the improved etiiciency throughout substantially the Whole range of frequencies to which the device may be subjected. In the ideal case all of the energy delivered in the one form to the device isconverted to the other form and transmitted without loss to the terminal load. In practice this ideal is not attainable since it is impossible to avoid a certain amount of energy dissipation in the structural elements of the device. In addition, the reactances of the oscillating elements limit the frequency range of the oscillations that can be transmitted. By this invention, however, a close approximation to the ideal case may be realized.

In accordance with the invention the elements of an electro-magnetic energy converting device, such as a telephone receiver, and other elements associated therewith, are proportioned and arranged to constitute a uni tary transmission line, in which waves oi all the, desired frequencies may be transmitted Without reflection, either at the ends of the line or at the point at Which the conversion of the energy from the one form to the other is effected. The transmission line is of the type known as a broad band wave filter. the theory of which is given in articles by G. A. Campbell-and O. J. Zobel in the Bell System Technical Journal for November 1922 and January 1923 respectively. In part the filter is electrical and in part mechanical, and these parts are so coupled by the electromagnetic, driving means that the combination is in effect a single homogeneous-filter.

It has been found that, when the conversion of energy from the electrical to the mechanical form is effected-by means of an electro-magnetic motor element such as is ordinarily used in telephone receivers, the range of frequencies for which the transmission and conversion of energy can be effected with uniformly high efiiciency is limited by certain factors relating to the essential elements of the energy converting device, name ly the inductance'of the driving coil, the mass of the mechanical element, or armature, to which the mechanical force is applied, and the force factor, which is defined as the ratio of the force in dynes impressed upon the armature to thecurrent-in c. g. s. units, flowing in the driving coil. These factors, as will be shown later, determine not only the frequency range of free transmission but also the characteristic impedances of the electrical and the mechanical portions of the line, or, in other words, they establish the basis for the complete design of the'filter sys tern.

The invention'also provides, for electrical sound reproducers, an electro-magnetic driving device of a particular form in which an exceptionally high force factor is obtained with a small inductance in the driving coil and a very small armature mass, whereby the frequency range is extended upwards to include all of the frequencies essential to the accurate reproduction of speech or music. This motor element is of the moving armature type, the armature being actuated bv a fixed coil and polarized by a steadv magnetic field. Toobtain the greatest possib e force factor the pole pieces of the polarizing magnetare arranged to provide four poles, which are disposed in a crossed relation so as to produce a pair of oppositely directed fields transverse to the longer direction of the armature. The crossed pole pieces converge to the center of the driving coil thereby c'oncentrat- 0o armature for a given windin ing the field in a very small air gap and making it possible to use an armature of minimum size. Since the polarizing fluxes do not traverse the armature longitudinally 1t is possible to make the armature extremely light while at the same time the polariaing flux, and hence the force factor is maintained at a very high value.

The particular form of this invention hereinafter described in detail is embodied in a loud speaking receiver for converting electrical energy into sound waves. A high degree of efiiciency of conversion is obtained by providing means for making the mechanical energy output substantially equal to the electrical energy input of the system. This maximum transmission efiiciency has been obtained by designing the electrical and the mechanical portions in accordance with the principles of wave pro agation through lumped structures, and esigning the coupling element as hereinafter disclosed. The

adjustment and the arrangement of the parts of the coupling'element should be such as to cause the measured value of the force'k in dynes per ampere turn acting onthe armature to equal or closely approximate the value given by the equation where f is the critical frequency of the system, N the number of turns of the exciting winding, m the effective mass of the armature and L the inductance of the exciting winding. This equation gives the dynes per ampere turn of the energizing coil which are required for any given values of mass, inductance and frequencyrange if the incoming electrical wave energy is toall flow out as mechanical wave energy except for inherent iron, copper and frictional losses. The result expressed by this equation may be closely approximated by an electrical magnetic arrangement in which the pole pieces terminate at or ver near the center of the receiver winding or the incoming signaling current. This concentration witha reduction in the size of the usual air ga between its pole pieces and the armature h s resulted in a considerable increase in the force acting on the and exciting current- The armature has a so been made as small and light as possible consistent with satisfactory operation. g

The couplingbetween the mechanical and electrical systems of the receiver ismain} tained at a high efiiciency for practically the entire speech or music-frequency range by the arrangement and proportioning of the mechanical system to give a mechanical network having substantially uniform efli-' ciency for allfrequencies below a definite cutoff frequency. One difiiculty in maintaining a uniform efliciency for the mechanical network is that any difi'erence between the restoring force acting on the armature and the magnetic pull on the armature exerted the pole pieces acts as aseries elasticity in the mechanical network and tends to make the mechanical impedance variable with frequency. One waythis undesired effect may be eliminated is to pivot the armature at one end on a supporting elastic member having a convexly curved surface, the curvature of which may be determined so that the restoring force acting onthe armature increases with the deflection of the armature away from its middle position in the same manner that the magnetic pull on the armature varies has been found preferable in many instances to always have the restoring force acting on the armature slightly greater than the magnetic pull from the pole pieces in order "to prevent the armature freezing to the pole pieces.

When the motor element is used in a telehone receiver or loudspeaker, the receiver diaphragm is connected to the armature by a chain of elastic and mass members the function of which is to effect a transformation between the mechanical velocity of the armature and that of the diaphragm, and also to impress the driving force at a number of points distributed over the surface of the diaphragm so that the diaphragmmay be made to move substantially as a solid piston. This in one form is accomplished by having two circular ridges of different diameters on opposite sides of the diaphragm. 'A spider having arms clamped against one of the ridges is located on each side of the diaphragm and the two spiders are driven by the armature of the receiver.

I Referring to the drawing, Fig. 1 is a sectional view of this invention embodied in a.

loudspeaking receiver. Fig. 2 illustrates the method of supporting the armature at one end thereof. Fig.3 is a top view of the arma; ture and its elastic supporting member. Fig. 4 illustrates in detail the arrangement for se-.. curing the plunger-like efiect of the diaphragm. Fig. 5 1s a perspective view of the manner in which the pole'pieces are arranged with respect to the magnet, the energizing coil and the armature. Figs. 6- and 7 illustrate different views of an alternative armature and armature support.- Fig. 8 illustrates by an equivalent electricalim edance diagramth impedance relations existing in both the electrical and mechanical parts of-the loud speaking receiver, and Fig. 9 is'adiagram used in explaining-the characteristics .of the electromagnetic unit of the receiver.

to a parti of a horseshoe magnet 15 as shown more in detail in Fig. in which the polar projections 11 and 12 are of the same polarity and the polar projections and 13 are of the same polarity. The energizing winding 16 upon which the currents to be translated into sound waves are impressed is wound between the pole pieces 10 and 12 and pole pieces 11 and 13 as shown in the sectional view in Fig. 1.

In order that the armature may be made as light as possible, only the portion 17 of the armature which is located between the pole pieces is made of iron while the remainder of the armature is of some light material such as aluminum or duralumin. Between the mass of iron 17 and the opposite end of the armature, the armature has been cut away leaving only two side strips 18 and 19 except for a tongue 20 which as will be later described is employed for connecting the armature to the diaphragm of the receiver. Each of the side strips 18 and 19 is stiffened to a considerable degree by having their outer edges turned up at right angles to the plane of the armature to provide stiffening ridges. At the end opposite the mass of iron 17 the armature is supported between two pairs of curved surfaces 21, 22 and 23, 24. The curved surfaces 21 and 22 constitute projections from a metallic member 25 which is rigidly attached to the supporting block 26 by suitable screws 27. The member 28 from which the curved surfaces 23 and 24 project is fastened to member 25 by screws 29 which pass freely through apertures in the armature 14.

As will be seen more clearly from Fig. 2 the sides of the armature 14 are bent such that for the armature in its middle position, the armature is contacting with points on the curved surfaces 21 and 22 which are substantially in the same plane as the contacting points on the curved surfaces 23 and 24. Unless this precaution is taken, there may be some danger of the armature when vibrating to slide along the curved surfaces due to the fact that it is merely clamped between the curved surfaces without any rigid connection thereto. As shown in Fig. 3, the screws 29 preferably engage arms extending from the member 28 so that an elastic pressure may be exerted by the curved surfaces 23 and 24 against the armature, the pressure being capable of variation by the adjustment of screws 29.

The tongue 20 which is used in attaching the armature to the diaphragm is preferably bent so as to form a substantially knife-edge on one side and a groove on the other as shown in Fig. 1. The pin 33 which extends between the diaphragm and the armature is provided with a washer 34 having a knifeedge working in the groove in tongue 20, and a second washer 35 having a groove or slot into which the knife-edge formed by the bending of the tongue 20 may work. These two washers 34*and 35 are held in position by suitable nuts 36 and 37 on the threaded 33 is attached to the diaphragm may be understood by reference to Figs. 1 and 4. A

centrall located aperture in the diaphragm 40 enab es the pin 33 and the rubber bushing 41 to pass therethrough while the spider 42 is held against one end of the bushing 41 and against the circular ridge 43 on the diaphragm by a locking nut 44. A similar spider 45 located on the opposite side of the diaphragm is held against the bushing 41 and against the circular ridge 46 by a locking nut 47. This provides-an elastic mass coupling to the diaphragm of particular utility as will be hereinafter described. Another advantage resulting from the use of these spiders 42, 45 is that a diaphragm of smaller mass may be employed due to the'multi-contact driving system. The manner in which the diaphragm 40 is secured at the small end of a speaking horn 50 is not a part of this invention but may be accomplished in any of the ways known in the art. It may be accomplished if desired, by the arrangement disclosed in my Patent 1,730,425 of Oct. 8. 1929, in which the air chamber at the small end of the horn is especially designed to obtain an eflicient coupling between the diaphragm and the column of air to be vibrated thereby.

A modified armature and armature mounting are disclosed in Figs. 6 and 7 The alternating current coil 52 is mounted between the pole pieces 53, 54, and 56 in the same manner as disclosed in Figs. 1, and 5, pole pieces 54 and 55 being of like polarity and pole pieces 53 and 56 being of like polarit The armature 57 is supported at one end etween two spaced points 58 and 59 bearing against the one surface of the armature and a curved surface 60 hearing against the opposite side of the armature. The posts 58 and 59 which end in the bearing points against the armature are suitably fastened in a rigid manner to a supporting member 61. The member having the curved surface 60 is provided with a pluralityof arms 62 and 63 so that b the adjustment of the screws64 and 65 an e astic pressure of any desired degree maybe exerted against the armature 57 by curved surface 60. Due to its trough-shape, the armature at its point of support has the curved surface bearing thereagainst at a'point between the supporting member 61 and the place where contact is made with points 58 and 59. This arrangement of the rocking surface on alignment with all pivoting points has been found desirable in order to prevent slipping between the rocker and its bearing surface.

The armature 57 between its point of sup- .port and the pole pieces of the magnet is preferably channeled in order to stiffen the armature. The portion between the pole pieces of the magnet is flattened in order that -the air gaps between' the armature and the pole pieces may be made as small as possible. The portion of the armature beyond the ole pieces 53 to 56 comprises a flexible ree 68 to which is fastened a pin 69 which leads to the diaphragm or other sound radiating memher.

As previously stated, the masses and elasticities of the various parts of the telephone receiver shown in the drawings should be arranged and given values to form a vibratory system of the filter type adapted to transmit uniformly and with small loss all of the frequencies in the range determined by the design of the motor element. Since the system as a whole constitutes a homogeneous filter it is convenient for the discussion of the principles underlying the designand the operation of the device, to represent the mechanical portion of the filter in the same conventional manner and by the same symbols as are used to represent the electrical filter. As is well know there is a correspondence between mass in the mechanical system and inductance in the electrical system, and a like correspond- ,ence between elasticity and the reciprocal of complete composite system in accordance with the electrical conventions. In this figure, an incoming electric line is shown connected by an input transformer 71 to a vacuum tube amplifier 72, the amplified currents from which by an output transformer 73 are impressed upon an inductance winding 74 which may represent the alternating current winding 16 of Fig. 1. The coupling between the electrical circuit and the mechanical circuit is illustrated schematically by an element 75 which represents the force factor as defined above, and serves to impress the electrical energy due to the current in coil 7 4-upon the mechanical portion of the system. The element 75 has no exact electrical equivalent, and is therefore represented in the drawing by a block. Its characteristics will be discussed in connection with Fig. 9. The series inductance 76 is the analogue for the mass of the armature 14, the series inductance 7 '7 corresponds to the combined masses of the spiders 42 and 45 and the inductance 78 corresponds to the effective mass of the diaphragm 40. The shunt capacity 79 corresponds to the elasticity of that part of the armature which couples the armature to the diaphragm, namely the tongue 20. Capacity 80 corresponds to the combined elasticity of the arms of the spiders 42 and 45 while the capacity 81 electrical capacity. Figure 8 represents the air chamber at the small end of the horn 50.

The coupling between the diaphragm mass or inductance 78 and the air column in horn 50, which may be made to have a substantially constant mechanical impedance represented by a resistance 82, is illustrated by a'cousling transformer 83 shown to be a stepown transformer since the diaphragm is preferably of greater diameter than the diameter of the small end of the horn.

For the purpose of simplicity, in the equivalent circuit of Fig. 8, the ratio of the lever between the armature and the spiders has been assumed to be one to one. In practice, it can have any desired ratio and, if other than unity, would be represented in Fi 8 by the inclusion of a non-unity ratio trans ormer .in the equivalent circuit.

The characteristics of the electro-mechanical transformer element of the receiver will 'now be considered. Equations in terms of the theory of electrical wave filters will be derived and it will be shown how the element may be advantageously joinedto the electrical 1 [and mechanical portion of the complete sys- We have given an electro-mechanical element, consisting of a pure inductance coupled to a mass by magnetic coupling. The transmission properties of the system depend upon the two characteristic impedances of the system corresponding respectively to the electrical and the mechanical ends of the element. At the mechanical end the impedance defines the ratio of. an impressed vibratory force to the resultant vibratory velocity and so is a quantity dimensionally different from the electrical impedance although it is analogous thereto when'the two modes of motion are compared. The characteristic impedances are defined as the impedances at each end re spectively when the other end is connected to an impedance equal to the characteristic impendance for that end, in other words, when the connected impedances are such that there is no reflection at the ends. The motor element is shown schematically in Fig. 9 in which the connected impedances are assumed to have the values defined above. The impedance at the electrical end of this type of structure is given by where Z is the damped impedance and the (FM term -1s the motlonal impedance. The

is the ratio of the mechanical force in d es on the armature per unit current in the driving coil. By the theory of reciprocity it also defines the E. M. F. induced in the exciting winding by unit velocity of the armature. The dimensions of the force factor are such that its square has the same dimensions as the product of an electrical and a mechanical impedance. The square of the force factor divided by a mechanical impedance is therefore an electrical impedance. The significance of the individual factor a and M will appear later. The properties of the electromagnetic force factor are described in an article by R. L. Wegel on Theory of magnetomechanical systems as applied to tele hone receivers and similar structures, A. I. E. Journal vol. 40, page 791, October, 1921, in which it is shown that the magnitude is constant at all frequencies and that the mechanical force is in phase with the electric current.

The quantity Z is the total mechanical impedance opposed to the impressed mechanical force by the inertia of the armatureand the mechanical system connected thereto, represented by Z The impedance of the armature, being due to inertia, has the same type of variation with frequency as the reactance of the driving coil, and moreover is reactive in the same sense, its tendency being to cause a lag of the vibrational velocity with respect to the driving force. The ratio of the armature impedance to the coil impedance is therefore a constant real quantity.

The form of Equation 1 expressing the im-- pedance of the system as measured at the driving coil terminals is the same as that for the impedance of a loaded transformer. This suggests that to avoid reflections at the ends of the system the impedances to which the motor element is connected should be of similar type, or, in other words that the two characteristic impedances are necessarily related by a constant factor, the values of which depend uppn the coeflicients of the motor element. egarded as a transformer the motor element comprises two elements having reactance of the inertia type, namely,- the coil and the armature, which are coupled together by the electro-magnetic effect represented by the force factor. The ratio of the mechanical reactance of the armature to the electrical reactance of the coil defines a transformation ratio analogous to the transformation ratio defined by the inductances of the windings of an electrical transformer. This ratio also defines the relative values that the connected impedances should have to avoid. reflection effccts, and consequently is also the ratio of the two characteristic impedances. It should be noted that while the ratio has a constant real value it isznot a simple numeric, since it expresses the relationship of two different quantities, namely an electrical impedance and a mechanical impedance.

Denoting the ration of the armature mass in ms to the coil inductance in c. g. s. units a, and making use of the fact that a must a so be the ratio of the two characteristic impedances it follows that Z3 a Z Z 1 Substituting these values in Equation (1) it follows that and which ma be solved for Z and Z,.

From t e foregoing it is seen that the factor M corresponds to an electrical impedance; it is the mutual impedance representing the coupling between the coil and the armature as it appears in the electrical part of the circuit. This mutual impedance differs from. that of an ordinary electro-magnetic transformer in that it is constant at all frequencies and has a real value in the complex plane.

Since by definition of aM, the force induced in the mechanical portion of the net-- work is 1, (1M, we have The ratio transfer constant 9 of the section. We have Since Z =j21rfL f being the frequency, the quantity under the square root sign in Equations (4) and (5) becomes zero at a frequency fc, defined by which will be designated the cut-off frequency. Below this frequency the quantity under the root sign is positive and less than unity and 6 is a pure imaginary quantity 1ndicating that the oscillations are not attenuated. Above the cut-off frequency 0 has a real value and indicates that attentuation takes place. In the two ranges the values of 0 may be more conveniently expressed as follows:

" W "it? (ill f=sinh 6* which may be put into more convenient form as follows Below cutoif e=0+ j sin 5; 7 Above cutolf 0=cosh (8) We can now write formulae (2) and (3) as follows This gives very important information. The network under consideration is seen to have the same image transfer constant at all frequencies as a half section of low pass filter and the same characteristic impedances at all fr uencies as a full section of the low pass fi ter. There is no electric network which is its equivalent in both respects.

Since the structure considered above has the typical low pass mid-series impedance characteristic, it may be joined without reflection at mid-series to other low pass strucnetic circuit of the receiver considerably.

- since the value of aM is unchanged and with twice the number of turns in the receiver windin it is much easier to obtain the predetermined required force on the armature per unit current in the winding.

A condenser 85 having one-half the capacity of a full section is connected across the receiver winding, so as to provide a midshunt termination for connection to the output transformer 73. We know from the theory of filters that the impedance Z of the network looking to the right from pomts 86, 87 is The initial or zero frequency impedance 'is therefore M, and followmg the general practice in filter design the impedance of the amplifier looking to the left from points 86,

87, should be M. This is for the reason that the amplifier impedance is substantially a constant resistance and it will be noted from Equations (11) or (9) that the characteristic impedance of the filter departs very little from the value M except for a small range of frequencies near f Similarly, the initial or zero frequency value of the characteristic impedance at each mid-series or mid-shunt point of the mechanical filter is (1 M. The resistance 82 should therefore have the value a f M in which f represents the impedance transformation ration of the transformer 83, that is, the ratio corresponding to the transforming effect of the air chamber coupling the diaphragm to the horn.

Calling the ca acity of condenser 85, C/2, the stiffness in orce per unit displacement of each of the elasticities 79 and 80, s, and each of the masses 76, 77 and 78, m, all in absolute c. g. s. units, we find from the simple formulae for low pass filters that:

a 1 0/2 ==mM=qrf L 1 1 1 m=aL and g=m=m The stiffness of the last elasticity 81 is, of course, 28 and its reciprocal, corresponding to capacity, is

2S 2a 1r f L From the value of M, if we call the initial characteristic impedance of the electrical portion of the circuit Z we have Z =M=1rf L Calling the initial characteristic impedance of the mechanical network Z',,, we have Z' =a M=a n-f L=-;rf m.

It is seen, from the above that the receiver with its associated electrical network forms the equivalent of an electrical line, such as that described in the U. S. patent to Campbell, No 1,227,113 of May 22, 1917, which transmits with uniformly negligible attenuation all frequencies below a cutoff frequency While suppressin frequencies above the cutofi? frequency. he mechanical portion of this line has substantially all the well known attributes and capabilities of electrical filters, e. g., values can be assigned to its several elements for causing the line to have the required cut-off frequency and the required characteristic impedance. \Vhile the filter is shown terminated at mid-shunt at both ends, it can have other terminations which may, for example, be desirable on account of the nature of the impedance to be connected thereto. Furthermore non-unity ratio transformers, mechanical or electrical, as the case may be, may be inserted for changing the required characteristic impedance 0 a portion of the line, as for example, to enable the use of masses or other elements having more convenient values from the practical standpoint.

If the electrical and mechanical network be designed as above described, the speech or other currents in line will flow without substantial transmission loss to the receiver winding 74 and after conversion into mechanical energy will flow without substantial transmisslon loss through the mechanical vibratory system and be translated into sound waves.

In the preceding analysis of the general principles of the invention it has been assumed that the electrical and mechanical quantities are all in c. g. s. units. This avoids the use of multiplying factors in the equations. In the part that follows, dealing with the application of the principles in the practical design of a telephone receiver the electrical quantities are in units of the practical system, amperes, ohms, etc. while the mechanical quantities are measured in the absolute system.

The conditions under which the telephone receiver of Fig. 1 may be connected to an electric line to form a combined electrifialmechanical system of high efficiency may be summarized, first, that the electric line working into the receiver element should have an impedance Trf L, where f is about 10% above the important speech frequencies and L is the damped receiver inductance; second, that the mechanical line into which the receiver works should have an impedance approximating qrf m, where m is the effective mass of the armature and third, that the force in dynes acting on the armature should have a value per ampere turn of the energizing winding of where N is the number of turns in the energizing coil. p

The third condition stated above is arrived at as follows: When the system is connected to terminal impedances equal to the characteristic impedances, or to fixed resistances equal to the initial characteristic impedances the total impedance as measured at either end is substantially constant throughout the transmission range, and'is a resistance substantially equal to the initial characteristic impedance corresponding to the end at which the measurement is made. The impedance at the driving coil terminals is substantially equal to Z and the mechanical impedance, driven by the armature is substantially Z',,.

The condition that all of the energy put in.

electrically should appear as mechanical energy may be written Since I and Z are assumed to be measured in the practical unitswhile the force (IaM) and Z are in absolute c. g. s. units the factor 10' is necessarily introduced. The force factor aM is proportional to the number of turns in the driving winding and may be expressed as kN, Where N is the number of turns and 7c is a constant. The factor in measures the force in dynes on the armature per ampere turn of the driving Windin Substituting these values and the values a ready given for Z and Z in the above equation gives t miv This last equation gives the dynes per ampere turn which are required for any given values of mass, inductance and frequency range if the incoming electrical Wave energy is all to flow out as mechanical wave energy except for iron, copper and frictional losses.

An analysis of the physical factors involved shows that it is physically possible to construct a magnetic motor element which meets the conditions for the equations set down in Equation (a). To facilitate the design procedure a formula for the factor k in terms of the dimensions of the magnetic circuit of the particular type shown in Figs. 1 and 5 will be developed. It is to be noted that in this type of structure the armature lies in an air gap between four magnetic poles and is magnetized longitudinally by the exciting coil. The four poles of the polarizing magnet are so arranged that, under the magnetizing action of the coil, the armature is subject to a lateral force only, each pole aiding the others in moving the armature in the lateral direction. The total force is therefore four times that due to any one pole. If B denotes the flux density due to the polarizing magnets in any one of the four air gaps, the cross sectional area of which will be denoted by A, and if 4 is the total flux produced in the armature by the exciting coil, then the resultant flux density in the air gap will be equal to I depending upon whether the two fluxes are aiding or opposing. The component of the force due to one pole is equal to fore given by F= =kNI (b) 21r The flux may be determined in the usual mannerfrom the magneto-motive force, defined as 4 1r 1 N I and the reluctance R of the magnetic path traversed by the flux Only the air portion of this path need be considered, and since there are four air gaps arranged in seriesparallel the total reluctance is simply that of a single gap, and is approximately 1.2 times the ratio of the gap length to its-crosssectional area. -The value of the flux 4/ is given by the equation,

""16 R which when combined with Equation (6) gives 2B Making use of this expression a typical value of k can be estimated. The flux density B may conveniently have the value 7500 lines per cm. and the length and crosssectional area of each air gap may be respectively .018 cm. and .063 emf, corresponding to a };gluctance R equal to 0.34. These values substituted in the above formula give Ir: 4400 dynes perampere turn. (0)

Equation ((1) relates the cut-off frequency of the system to the constautk and to the electrical and mechanical coe flicients'of the system. Since the quantity which appears therein is determined almost entirely by the geometrical configuration of the magnetic circuit, it is possible to obtain an expression relating the cut-off frequency solely to the factor k and the mechanical dimensions. Such an expression is most convenient for design purposes.

Assuming that the armature flux is a constant fraction, denoted by d, of the total fllpx generated by the exciting coil it follows t at The elimination of I, L, N and b in M115.

equation and Equations 11 and 12 gives For high quality speech and music transmission, f may have'a value of 6,000 cycles. A practicable value for d is .8 and for B is 7,500 gausses, although values as low as 4,000 gausses will be satisfactory. Substituting these values and the value for; is from Equation $11)) in the last equation and solving for m t e mass of the armature, the mass of the armature would be .1 gram. This armature mass corresponds to the air gap dimensions for which the typical value of is was computed. By converging the poles in the manner of Figs. 1 and 5 the area of the armature need not exceed twice the area of a single pole face, or, for the particular example, .125 cm. The average thickness would be 0.1 cm.

The receiver structure illustrated in Fig. 1 is of a type in which the value of k, that is, the force in dynes acting on the armature per ampere turn of the exciting winding may ave a measured value closely approximating the theoretical value given by the Equation (a) above.

The alternatin flux circuit comprising the rectangular mem ers fastened to the magnet 15 and ending in the ole pieces 10 to 13 may be built of laminate magnetic material such as silicon or nickel steel. These rectangular members should be made of large area but as short as possible in order to reduce to a small value the reluctance of the alternating fiux circuit. The pole pieces 10 to 13 are crisscrossed as shown in Fig. 1 to minimize alternating leakage flux and to make the length of the armature flux ath a minimum. If the steady flux in the magnetic system of Fig. 1 is 7,500 gausses and the fraction of the linkages due; to pole face alternating flux is .7 and the mass of the armature .1 gram, the measured value of k, that is, the measured value of the force in dynes acting on the armature per ampere turn of the coil will closely approximate the value given in the above equation whereby a substantially free transfer of the speech energy band from to 6,000 cycles, for example, may be had from the electrical to the mechanical system with a very high degree of efliciency.

In Fig. 1, the arrangement disclosed provides a spring restoring force which balances the steady magnetic pull for all armature positions. The specific compensating means disclosed is an improvement of the arrangement described and claimed in my *Patent 1,562,165, Nov. 17, 1925, on acoustic devices. As described briefly above, the compensating means is produced by clamping one end of the armature 14 between two curved surfaces 21 and 23. \Vhenever the armature is displaced from its neutral position, the armature will separate the two curved surfaces 21 and 23 working against the elasticity in the arm connecting the curved surfaces 23 and 24 so that the. armature will contact with points on the curved surfaces removed from the center thereof. This will produce a restoring force which may be made to vary with the magnitude of the armature displacement in such a manner that the restoring force ofall positions substantially balances the steady magneticpull exerted on the armature. In practice, the restoring force for the maximum displacement of the armature should be made slightly greater than the magnetic pullon the armature in order to prevent the possibility of the armature freezing to the pole pieces. The curved surfaces 21' to24 will not necessarily be arcs of a circle but will be made to have such a curvature as to produce the compensation desired for all armature positions.

It will be understood that the restoring force for changes in flux due to alternating currents in the receiver winding is supplied by an elasticity which is effectively in shunt to the line of propagation,i.e.by thetongue 20.

It is, of course, understood that the system disclosed in Fig. 1 may be employed with equal efficiency for translating electrical energy into mechanical energy or mechanical energy into electrical energy so that the device disclosed may be employed either as an electromagnetic receiver or an electromagnetic transmitter. The magnetic motor element is also capable of use in other magnetic systems as will be apparent to those skilled in the art. It is also to be understood that the specific arrangements of the parts of the magnetic system'may be modified in various ways while still obtaining the high efiiciency characteristic of the particular type described in detail above for illustrative purposes.

A typical receiver structure embodying this invention will now be described in detail to illustrate one way in which the object of this invention may be attain I In designing a cries-cross p0 e motor ele-' ment to worln through a multi-point drive diaphragm ifito a horn, the most straightforward procedure is to first design the multi-point drive diaphragm, next the motor element, and finally the coupling between them.

The multi-point drive diaphragm will be assumed to Work into a horn of len th 36", mouth diameter 22", small end iarneter .69 and having a logarithmic outline. The impedance of this horn will be considered to be that of an infinite tube of the inside diameter of the small end. This assumption will be fairly accurate forwave lengths of sound greater than twice the mouth diameter of the horn. Since the impedance of a straight tube of infinite length and uniform crosssection is 43 mechanical ohms per cm. (dynes per cm. per unit velocity of air displacemelnt), the impedance of the horn is assumed to e:

104 mechanical ohms. (1)

The dimensions for the diaphragm may be as follows: clamping diameter 1% inches, width of flat edge portion inches, diameter of outer driving ring 1.25 inches, diameter of inner driving ring inches. This diaphragm, driven as it is, in Fig. 1 acts approximately as a plunger of a diameter of v 1.69 inches=4.29 cm.

It will be approximately correct to consider the sound pressure uniform throughout the coupling chamber between the diaphragm and horn, in which case the mechanical impedance looking away from the chamber should be made proportional to the squared value of the areas working into the chamber.

A =area of small end of horn.

the horn assumed here A 7r .876 =2.42 cm.

Solving this equation the required value of Z is found to be -3730 h 111 242 mec amca 0 ms. (3)

The mechanical transmission line asso- For ciated with the diaphragm may be required to transmit frequencies to approximately 5,500 cycles, to'take care of music transmission which r uires a critical frequency of about 6,000 cyc es, and hence the permissible diaphragm mass is:

Associated with the diaphragm in the mechanical transmission line is a full section shunt elasticity-on the receiver side which should have the value 8=1r f m=1r X 6000 X .198=

70.6X 10 dynes per cm. (5) and a half section on the horn side i. e. the

value of elasticity the coupling air chamber should be 2X70.6 10 dynes. The volume of the chamber should be Where S=Youngs modulus for air= 1.5X 10 dynes per cm. at atmospheric pressure.

The shunt elasticity on the armature side of the diaphragm is divided into a number of parallel elasticities pressing against the diapxlllragm at points scattered over its area.

ith this multi-point drive, it is possible to use very thin and light diaphragm material and also make the natural periods of the portions of the diaphragm between driving points lie well above the important voice freuencies. It is thus made possible to match t e impedance of the diaphragm into that of a horn, over the voice range'without making the sound passages in the coupling air chamher and into the horn constricted and without having high sound pressures in this chamher, with resulting non-linear distortion. A convenient way of obtaining the elasticities is in the form of two spiders, as shown in Fig. 1. The mass of the diaphragm associated with the center spider will be is 1.23 cm. 'm'=mass per sq. cm. of diaphragm area. For .002" duralumin m'=.0137 gm. Choosing eight prongs as a good number to distribute the contact pressures and driving force well the mass per prong is m= =.0082 gm. 8

The required elasticity. per prong is then The mass associated with the outerspider is m=1r(r )m'=1r(2.19 -1.23 .0137= Assuming 8 prongs per spider the required elasticity per prong is i .144 s.=ar f m=1r 6000 X 8 6.41 X 10 dynes per cm. (12) Assume the spider to be of .012'-"=.0305 cm. duralumin for which Youngs modulus= 7 X10 'and the prongs to be %"=.635 cm. long. The required width is then The natural periods of the prongs of these spiders must lie well above the voice frequency range. For a duralumin reed clamped at one end and supported at the other the natural period will have approximately the value f.; P cycles. (13) Where f =fundamental period in cycles per sec.

t =thickness in cm. Z =length in cm.

For the small spider 476, 16500 cycles. i

For the large spider f0 1 1200 cycles.

The center discs of the two spiders plus the coupling rod constitute the second mass. This should preferably have the same value as the diphragm i'. e., 198 gm. This will remove about half the material of the larger disc. The remainin framework will require stiffening by embossing in order that it may all move together asa mass.

The next matter is the design of a magnetic motor element to drive this diaphragm and born. The problem is most conveniently divided into three parts; the magnetic, the mechanical and the electrical. The magnetic circuit, to be ideal, should have a very high, steady flux density in the air gap, a very high percentage of the alternating flux circuit reluctance localized in the air gap and a very high percentage of the alternating coil linka e flux should pass through the pole faces. ith available magnetic materials about 15000 lines per cm. represents the highest practicable value. The magnet required to produce this flux will be determined after the air gap is fixed. To make the reluctance of the iron part of the alternating flux path small compared with that of the air gap, its length must be made small and its cross-section large. Assuming an alternating current permeability of 300, a cross-section of iron four times that of the pole face except very near the pole tip and a mean length of path of .75=1.91 cm., the reluctance of the iron path is approximately Where R =reluctance A,,=area of ole face. The reluctance 0 the air gaps, for two .014=.0356 cm. gaps in series is Hence for this design the fraction of the total reluctance in the air gap is r To make the fraction of the linkage flux passing through, the pole faces as large as possible, the alternating current flux pathshould, as far as possible, not be saturated by steady flux and the alternatin current flux path in the armature should short, the air gap should be located as nearly as possible at the center of the alternating current coil andthe reluctance of the iron compared with the air gap should be small. To minimize the effect of the steady flux on the alternating flux circuit the pole pieces should have three to four times the cross-sectional'area of the pole faces oersteds.

oersteds.

, and the steady flux should pass through the armature in its thinnest dimension and at right angles to the alternating flux pa-th. To

make the alternating flux path in the armature of minimum len h the pole faces must be brought close toget er as in the criss-cross pole receiver. The position of the coil in this receiver places the air gaps nearly at the ance at all fr center of the coil where the magneto motive force is most concentrated. In general, they should notfbe brought so near, however, that the flux passing directly between them is any considerable part of that passing through the armature. With air ga s of .007 under pole faces .250" .040, a istance between pole faces of .025" may be expected to give about coil linkages due to pole face flux.

Taking up next the mechanical side of the motor element, the most important matters are; the ratio of force per ampere turn to armature mass and the power capacity of the motor element. The mechanical limit on power capacity is set by the mass of the armature and its possible amplitude of vibration before it either becomes saturated with flux or strikes the pole pieces. The force per ampere turn depends upon the three magnetic factors; steady flux density, percentage alternating flux circuit reluctance concentrated in the air gap and ercentage of coil linka es due to pole face Eux. Assuming values or these of 7,500, and 80%, for free flow of energy between the electrical and mechanical circuits over the voice range of frequencies, the. mass of the armature per cm. of pole face area in parallel should have the value of .4 gm. if there are two .014 air gaps in series. If the air gaps are reduced by some -factor N, the mass should be increasd by the same factor. To reach this value of armature mass requires the use of a very thin armature and a very small air gap. Assuming pole faces .25 X .080" with a distance between pole faces of .050, the pole face area is: A .25 X .080 X 2 1040 in. =.258 Cm. The correct armature mass is then m= .258 X 4 103 gm. Assuming an armature .25" wide, of uniform thickness for the .050" between the pole faces and then tapering to zero thickness at the outer edge of the pole face, the mass for a .024" armature would be m .25 X .024 X (050+ .080) X p 2.54 X 7.8 .1 gm. With an armature cross-sectional area of 6X10 in. and a pole face area of 20 10 in. the displacement before the armature will become saturated with flux will evidently be only about one third of the air gap. With this armature the possible amplitude will be about .005 in. Assuming this amplitude at 200 cycles the power output would be P= (.005X2.54X2-n-X200)X.1X1rX6000= .48X 10 ergs per second=.048 watts. Taking up finally the electrical side of the motor element, the important matters are the size of the winding and its resistance. The resistance of the winding including iron losses should be less than 20% of the reactuencies from 100 to 6000 cycles. The coil shou d besmall in physical dimensions in order that the flux linkage other than those due to pole face flux may be small. Assuming 80% reluctance in the air gap and 80% of linkages due to pole face flux the inductance per 100 turns for two pole faces in parallel having a combined area of 2 X .25" X .080" 04' .258 cmP,

Z 000 .458 X10 8.65 ohms.

If the number of turns is increased to 107 0 then the line impedance should be 1070 2 Z=(- 8.6o- 1000 ohms. The shunt capacity for this impedance is given by The coupling between the armature and the spiders which drive the diaphragm remains to be designed. The mass of the armature has been fixed at .1 gm. and that of the diaphragm at .200 gm. If vibratory energy is to flow freely from one to the other the ratio 2 .0265 mid.

of their velocities must be inversely as the square root of the ratio of their masses. If the armature is carried by a light stiff duralumin frame which pivots at a point away. the diaphragm spiders should be attached at a distance from the pivot point of Between the frame and the spiders there should be an elasticity of 8 1r f .200=71.3 X 10 dynes per cm.

Assuming the frame made of .010 .0254- cm. duralumin, a tongue .135" .343 cm. Wide and 3/32" .238 cm. long Will give an elasticity of:

71.3X 10 dynes per cm.

The invention claimed is: 1. An electromagnetic system comprising a magnet having pole pleces, a mechanical vibratory system comprising an armature arranged in the field of said pole pieces, and an alternating current winding in operative relation to said armature, said armature having a mass of not more than one gram per square centimeter of pole face area, more than of the alternating flux linkages being produced by the flux passing through the pole faces to the arn'iature.

3.2. An electromagnetic system comprising a magnet having pole pieces, said magnet having a steady flux density in excess of 5000 lines per square centimeter of pole. face area, a mechanical Vibratory system comprising an armature arranged in the lield of said pole pieces. and an alternating current winding in operative relation to said ari'rniture, said armature having a mass of not more than one gram per square centimeter at pole face area, more than 50% of the lines of force produced ny the alternating current in said coil being effective by passing through the pole faces to the armature. 3. An electronnignctic system comprising a magnet having pole pieces a mechanicai viln'atory system compri ing an armature arranged in the field of said pole pieces, an alternating current winding in operative relation to said armature. said armature hav' ing a mass of not more than one gram per square centimeter of pole face area, the air gap between each pole face and said armature being less than .015 inches and an alternating flux circuit comprising said armature and said pole pieces. more than 75% of the re luctance of said alternating ilux circuit being comprised in the air gaps between said pole faces and said armature.

4. An electromagnetic. system comprising a magnet having pole pieces, a mechanical vibratorv system comprising an armature arranged in the field of said pole pieces, an aiternating current winding in operative relation to said armature, said armature having a mass of not more than one gram per square centimeter of pole face area. and means for creating a restoring force acting on the armature for substantially equalizing the pull of the steady magnetic flux exerted on said armature by said pole pieces for all armature positions.

5. An electromagnetic system comprising a magnet having pole pieces, a mechanical vibratory system comprising an armature arranged in the field of said pole pieces. an alternating current winding in operative relation to said armature said armature having a mass of not more than one gram per square centimeter of pole face area, more than 50% of the lines of force produced by the alter mating current in said coil passing through the pole faces to the armature, said vibratory system being arranged and adjusted to present a substantially pure resistance over substantially the entire speech range.

6. An electromagnetic systorn comprising a magnet, an armature in the field of said "magnet, an alternating current winding in operatiye relation to said armature, said systern belng arranged such that the force in dynes per ampere turn of the current coil acting on the armature is greater than 10% of the optimum value given by the equation where f is the critical frequency, N, the number of turns of said coil, m, the effective mass of said armature and L the damped inductance of said coil.

7. An electrical device comprising an electromagnetic system having a magnet with pole pieces, a mechanical vibratory system comprising an armature arranged in the field of said pole pieces, an alternating current winding in operative relation to said armature, said armature, having a mass of not more than one gram per square centimeter of pole face area, the air gap between each pole face and said armature being less than .015 inches and an alternating flux circuit comprising said armature and said pole pieces, more than 75% of the reluctance of said alternating flux circuit being comprised in the air gaps between said pole faces and said armature, and a mechanical transmission line having a substantially constant impedance over a wide frequency range for coupling said armature to the air.

8. A translating device comprising an electromagnetic system having a magnet, pole pieces attached to said magnet, an armature supported between said pole pieces, an alternatlng current winding supported in operative relation ,with said armature. said magnet, pole pieces, armature and winding being arranged and constructed to cause the electrical power in said winding to be of the same order of magnitude as the mechanical power in said armature when said device is operated.

9. A translating device comprising an electromagnetic system having a magnet, an armature supported within the field of said magnet, an alternating current winding sup ported in operative relation to said armature, said magnet having attached thereto a short alternating flux path of magnetic material with a plurality of pole pieces of each polarity concentrated to give a transverse steady magnetization .of said armature to maintain the armature flux path over only a small portion of the length of said armature thereby causing the fraction of alternating flux lines of force passing from said pole Ipieces to said armature to be greater than fty percent the total number of alternating flux lines of force developed, by said winding, the major portion of the mass of said armature being concentrated between said pole pieces.

10. An acoustic device comprising a magnet, anal-mature in the field of said magnet,

an alternating'current coil in operative rela-'- tion to said armature, a flexible member of extensive area attached to said armature, the measured value of k, the force in dynes per ampere turn of the current coil acting on the armature being of the order of magnitude given by the equation where f is the critical frequency, N is the number of turns of said coil, m the effective massof said armature and L the damped inductance of said coil.

11. An electromagnetic device comprising a magnet, pole pieces attached to said magnet, an armature between said pole pieces, an

alternating current coil in operative relation to said armature, said pole pieces being concentrated to operate only on the small portion of said armature to give a large percentage of linkage between said armature and said pole pieces for alternating flux, the effective mass of said armature being reduced to such an extent that the measured value of In is of the same order of magnitude as that given by the' equation k arn where k, N, L and m are defined in claim 10.

13. A telephone receiver comprising a magnet, an armature in the field of said magnet, an alternating current coil in operative relation to said armature, a vibrating member attached to said armature, said coil, armature and magnet being so arranged and proportioned that when said device is in operation, the mechanical power in ergs per second of the vibrating parts of the system is of the same order of magnitude as the electrical power in ergs per second, in said coil except for inherent iron, copper and frictional losses.

14. An electromagnetic system comprising an alternating current winding, a magnet having two pairs of opposed pole pieces terminating substantially at the center of and within said winding, and an armature supported between said pole pieces.

15. An electromagnetic system comprising a solenoidal winding, two pairs of opposed pole pieces terminating within said winding, and an armature within said coil and supported between said pole pieces and controlled by said winding.

16. An electromagnetic system comprising an altern ting current winding, a magnet having two pairs of opposed pole pieces tertransverse the axis of said solenoid, and a diaphragm attached to said armature.

18. An electromagnetic system comprising an alternating current winding, an' armature parallel to the axis of said winding and adapted to move transverse the axis thereot,

and a magnet having pole pieces projecting into the interior of said winding.

19. A telephone acoustic device co1nprismg an alternating current winding, an armature parallel to the axis of said winding and adapted to move transverse the axis thereof, a magnet having a pair of pole pieces projecting into the interior of said winding at each end thereof, and a diaphragm attached to said armature.

20. An electromagnetic system comprising an armature, a magnet having a pair of pole pieces of opposite polarity angularly spaced on one side of said armature, a second pair of pole pieces of opposite polarity angularly spaced on the opposite side of said armature, and an alternating current winding Wound between the members of said pairs of pole pieces.

21. An electromagnetic system comprising a magnet having a pair of pole pieces of opposite polarity angularly spaced on one side of said armature, a second pair of pole pieces of opposite polarity angular-1y spaced from each other on the opposite side of said armature, an alternating current winding held between the pole pieces of said pairs and a sound radiatin member of extensive area attached to sai armature.

22. An electromagnetic system comprising an armature, two angularly dis osed pole pieces arranged adjacent one side of said armature, two angularly disposedpole ieces arranged adjacent the op osite side 0? said armature, that part of sai armature located within the magnetic field produced by said pole pieces bein of graduated thickness, the ends of said po e pieces adjacent the armature being formed to give an air gap of substantially constant width between each pole piece and the armature, and an alternating current winding between the sets of pole pieces and surrounding said armature.

23. An electromagnetic system comprising an armature, a magnet having a plurality of pole pieces at opposite sides of said armatni and converging toward the center thero'd'. two of said pole pieces on one side living i-t opposite polarity, two of said pole pieces 0: the other side being of opposite polarity earl. of said pole pieces being directly opposit" a pole piece of the opposite polarity, and an alternating current winding cooperating with said armature.

24. An electromagnetic system comprising an armature, a magnet having a plurality of pole pieces at opposite sides of said armature and converging toward the center thereof, two of said pole pieces on one side being of opposite polarity, two of said pole pieces on the other side being of opposite polarity. each of said pole pieces being'directly opposite a pole piece of the opposite polarity, and an alternating current winding surrounding said armature and located between pole pieces of opposite polarity on each side of said armature.

25. An electromagnetic system L'UIllpllSlhQj an armature, an alternating current winding surrounding said armature, a magnet having a plurality of pole pieces of like polarity projecting into said winding at opposite ends thereof, and on opposite sides of said ar1nature, ,a plurality of pole pieces of diti'erent polarity from said first plurality of pole pieces projecting into said winding at opposite ends thereof and on opposite sides of said armature, and a sound radiating member of extensive area attached to said armature.

26. An electromagnetic system comprising an armature stiffened against bending over an appreciable portion of its length, supporting means for said armature operating on said stiffened portion, another portion of said armature being flexible and a diaphragm attached to said armature.

27. An electromagnetic system comprising a magnet, an alternating current winding, an armature cooperating with said winding and said magnet, said armature being stiflened over a major portion of its length, another portion of said armature being of snfiicient flexibility to provide a shunting elasticity of 'a value determined by the effective mass of said armature to form one section of a mechanical network which in a plurality of similar sections would have a substantially con-r stant impedance for a wide range of frequencies below the cut-off frequency where the cut-off frequency fr is given by s S fa x m where s is the elasticity of the flexible part of the armature acting as a full shunt element and m is the effective mass of the armature acting as a half series element.

'28. An electromagnetic system comprisinga magnet, an armature, an alternating curliJil rent winding cooperating with said magnet and said armature, a corrugated member of extensive area, metallic means having a plurality of flexible arms contacting with a corrugation on one side of said member, a second metallic means having a plurality of flexible arms contacting with a corrugation on the opposite side of said member, and means for attaching said metallic means to said armature.

29. An electromagnetic system comprising a magnet, an armature, an alternating current winding for said armature. a vibrating member of extensive area having a plurality of ridges on said member describing a circular path about the approximate center of said member, metallic means having a plurality of arms clamped against only one pf said rid es near the outer ends of said arms, a secon metallic means on the opposite side of said member. said second metallic means having a plurality of arms clamped against a second ridge near the outer ends of said arms, and means for attaching said metallic means to said armature.

30. An electromagnetic system comprising a magnet, an armature, an alternating cur rent winding for said armature, a corrugated diaphragm, a spider having a plurality of flexible metallic arms contacting with a corrugation of said diaphragm on one side thereof, a second spider having a plurality of flexible metallic arms contacting with a second corrugation of said diaphragm on the opposite side, and a connecting rod between said spiders and said armature.

31. Sound radiating mechanism. comprising a corrugated diaphragm, spiders at opposite sides of said diaphragm having flexible arms contacting with respective corrugations, and common means for vibrating said spiders.

32. Sound radiating mechanism, comprising a concentrically corrugated diaphragm a plurality of spiders having flexible arms contacting with respective corrugations and common means for vibrating said spiders.

33. An electromagnetic system comprising a magnet, an armature, an alternating cur rent winding cooperating with said magnet and said armature. means for elastically supporting said armature at one portion thereof. pole pieces from said magnet acting on another portion of said armature, a spring member forming a unitary part of said armature,

a diaphragm, and a rod directly connecting said spring to said diaphragm.

34. An electromagnetic system comprising a magnet, an alternating current coil, an armature cooperating with said coil' and said magnet, members having opposed curved surfaces. one end of said armature being clamped between said curved surfaces and the other end being free to vibrate and a sound radiating member attached to said armature.

35. An electromagnetic system comprisin a magnet, an armature in the field of sai magnet, an alternating current winding in operative relation to said armature, said mag net having a plurality of pole pieces of 0pposite polarity on each side of said armature, said pole pieces of like polarity being crisscrossed.

36. An electromagnetic system comprising a magnet, an armature in the field of said magnet, an alternatin current winding in operative relation to said armature, said magnet having a plurality of pole pieces of opposite polarity on each side of said armature, said pole pieces being concentrated over a small portion of said armature, and being crisscrossed.

37. An electromagnetic system comprisin" a magnet, an armature in the field of said magnet, an aiternating current winding in operative relation to said armature, said magnet having a plurality of pole pieces of opposite polarity on each side of said armature, and concentrated over a small portion of said armature. the portion of said armature between said pole pieces being bevelled and having a mass greater than the mass of the remainder of said armature.

38. An electromagnetic system comprisin a magnet, an armature in the field of said magnet, an alternating current winding in operative relation to said armature, said magnet having a plurality of pole ieces of 0ppcsite polarity on each side 0% said armature. said pole pieces being concentrated around a small portion of said armature such that the surface area of portion of the armature traversed by alternating flux is substantially equal to the combined surface area of the ends of the pole pieces.

39. In combination an electromagnetic receiver, having the characteristic impedance of an electric wave filter and reactance connected to its electrical terminals for giving the combination a mid-section termination.

40. In combination, an electromagnetic receiver having the characteristic impedance of a low pass wave filter and reactance connected to its electrical terminals, for giving the combination a mid-section termination.

41. In combination, an electromagnetic receiver having the characteristic impedance of av low pass wave filter and reactance connected to its electrical terminals, for giving the combination mid-shunt termination.

42. In combination. an electromagnetic translating devicehaving the characteristic impedance of a wave filter the armature of said device serving as the equivalent of a series inductance in an electric wave filter, a diaphragm and means connectin said armature to said diaphragm, said iaphragm and said means comprising mass and elasticity having such values as to form at least one filter section having substantially the same characteristic impedance as said device.

43. In combination, a line, an electromagnetic device comprisim a magnet, an armature in the field of said magnet, an alternating current winding connected to said line and arranged to be in operative relation with said armature and a capacity connected in shunt to said winding and said line, said winding and said capacity having such values that a plurality of similar sections connected in tandem would form an electric network having a critical frequency near the upper range of important speech frequencies.

44. In combination, an electric line, an electromagnetic system comprising a magnet, an armature in the field of said magnet, an alternating current winding connected in se ries With said line and arranged to be in operative relation with said armature. and a capacity connected in shunt to said line and to said winding, said Winding and capacity having such values that an infinite number of similar sections connected in tandem should have a characteristic impedance substantially equal to the characteristic impedance of said line,

45. In combination, an electric line, an electromagnetic system comprising a magnet, an armature in the field of said magnet, an alternating current winding connected in series with said line and arranged to be in operative relation with said armature and a capacity connected in shunt to said line and to said winding, said capacity having a value substantially the same as that given by the equation:

where f is the critical frequency and L the inductance of said winding.

46. A sound radiating mechanism, comprising a corrugated diaphragm, a driving member therefor and a spider connecting said diaphragm and said driving member, said spider having flexible radial arms contacting with the ridge of a corrugation in said diaphragm.

47. An electromagnetic device for transforming electrical vibrations into mechanical vibrations or vice versa, comprising an electrical circuit having an impedance Z,,, and a mechanical vibratory system electromagnetically coupled thereto constituting a mechanical wave transmission line having a characteristic impedance Z, characterized in this'that the square of the force factor of the electromagnet coupling is substantially e ual to 10" times the product Z Z' whereby t e system is constituted a homogeneous line.

In witness whereof, I hereunto subscribe my name this 5th day of December, A. D

HENRY C. HARRISON. 

